Within twenty years of one another, Bacon and Descartes proposed cosmologies which relied heavily on matter theory. In both, the distribution of matter in the cosmos determined what centers of rotation there were, and rotating bodies were carried around by the motion of an all-encompassing celestial fluid in which they were embedded. But the role of matter theory in the two accounts is very different, both in motivation and in the level at which it is active in guiding physical theory. Matter theory in Baconian cosmology stands as a foundational discipline, being virtually constitutive of physical theory, as it had been for natural philosophers from Thales onwards, whereas in Descartes it is subservient to the needs of his optics and his mechanics. Comparison of the two cases shows how the role of matter theory came to be radically modified in seventeenth-century cosmology